Self-repairing interphase reconstructed in each cycle for highly reversible aqueous zinc batteries

Aqueous zinc (Zn) chemistry features intrinsic safety, but suffers from severe irreversibility, as exemplified by low Coulombic efficiency, sustained water consumption and dendrite growth, which hampers practical applications of rechargeable Zn batteries. Herein, we report a highly reversible aqueous Zn battery in which the graphitic carbon nitride quantum dots additive serves as fast colloid ion carriers and assists the construction of a dynamic & self-repairing protective interphase. This real-time assembled interphase enables an ion-sieving effect and is found actively regenerate in each battery cycle, in effect endowing the system with single Zn2+ conduction and constant conformal integrality, executing timely adaption of Zn deposition, thus retaining sustainable long-term protective effect. In consequence, dendrite-free Zn plating/stripping at ~99.6% Coulombic efficiency for 200 cycles, steady charge-discharge for 1200 h, and impressive cyclability (61.2% retention for 500 cycles in a Zn | |MnO2 full battery, 73.2% retention for 500 cycles in a Zn | |V2O5 full battery and 93.5% retention for 3000 cycles in a Zn | |VOPO4 full battery) are achieved, which defines a general pathway to challenge Lithium in all low-cost, large-scale applications.

techniques such as XANES spectra, in situ AFM, in situ fluorescence microscopy, GIXD and in situ Raman spectra were carried out deeply explore the action mechanism of C3N4QDs. However, electrochemical performance of both half cells and full cells are not excellent compared recent literature. Therefore, this work should be carefully evaluated after the follow suggestion are taken into consideration. 1. Carbon quantum dots are known as zero-dimensional carbon nanomaterials. You mentioned that C3N4QDs have a typical thin 2D morphology. Although AFM measurement indicates layerstructure, well-defined lattice fringes are not clearly observed in TEM image. The description may not suitable. If possible, please give related reference. 2. MD simulation was carried out to demonstrate the changes of solvation structure of Zn2+. How does C3N4QDs with an average lateral size of ~10 nm enter primary solvation shell of Zn2+? And what is the basis of molecular models of C3N4QDs? Related reference should be supplemented to confirm this conclusion. 3. As shown in Fig S8, when the concentration of C3N4QDs is 4 mg/mL, the smaller charge transfer resistance is obtained. Please give reasonable excuse. 4. Authors claimed that C3N4QDs can participate in metal coordination via intrinsic pore, therefore 185 Zn2+ ions are carried up at the same time. The pore structure of C3N4QDs should be given. Most important, this may contradict the previous MD calculation. The mechanism of C3N4QDs seems confusing, the detailed explanation should be added. 5. Real-time SEI reconstruction mechanism is demonstrated by in situ fluorescence microscopy observations. However, the fluorescent signal may not fully prove self-adaption and self-healing mechanism. The XPS results of electroplating and stripping to further demonstrate the conclusion. 6. SEM images indicate the successful suppression of Zn dendrites; however, cross-sectional images may more validly reflect morphology evolution of Zn. 7. The authors mentioned that C3N4QDs could effectively improve Zn2+ transfer number. And then you claimed that C3N4QDs slightly lowers the ionic conductivity and increase charge transfer resistance. Is there a necessary connection between the ionic conductivity and transference number? 8. The Zn||Zn symmetric cell with C3N4QDs exhibits excellent performance, however, the Zn||MnO2 and Zn||VOPO4 show poor cycling performance. And the rate performance of full cells is missing. Most important, the obvious activation process of Zn| C3N4QDs |VOPO4 could be observed, but the same phenomena is not found in blank group. 9. The electrolyte of half cells and full cells are different, especially for Zn||VOPO4.As we all know, electrolyte as vital component plays important role in electrochemical behavior. Different electrolyte means different property; therefore, it is necessary to use the same electrolyte.

REVIEWER COMMENTS Reviewer #1 (Remarks to the Author):
Dear editor, I read with interest the manuscript by Zhang et al. reporting on using a dynamic interphase formed by graphitic carbon nitride as an electrolyte additive to improve the performance of Zn metal anodes. The concept appears to be scientifically. The authors also used complementary characterization tools (those at beamlines) to investigate the process. Overall, this works appears to be of interest to the readership of Nature Comm, and I thus suggest that authors further revise the manuscript to address the issues as detailed below.

Manuscript ID: NCOMMS-22-07115
conclude so based on the TEM and AFM shown in Fig. S1~2. The lateral and the vertical dimensions of the particle are at a comparable level, which doesn't suggest a thin, 2D morphology. For example, see Fig. 6 for morphology of a more conventional 2D material at DOI: 0.1002/chem.200902478. Response: We would like to thank the Reviewer for pointing out this somewhat confusing definition bout 2D quantum dots. By definition, "dots" imply zerodimension equiaxed structures like spheres or cubes, controversial to the expression of 2D. Strictly speaking, there is no 2D quantum dots. As the Reviewer pointed out, 2D morphologies are typically used to describe thin (few-layer) materials with large lateral sizes, such as a piece of few-layer graphene with a lateral to axial (thickness) size ratio of over 10,000.
The situation becomes interesting when the lateral size of a piece of 2D material (e.g., graphene) was reduced to several nanometers, which is in the range of the size of quantum dots and therefore often called quantum dots. As the lateral size/thickness ratio decreases to ~10, technically the resulting materials are not typical 2D morphology anymore. However, the terminology of "2D quantum dots" is widely used in literature to describe quantum dots derived from 2D materials and are not equiaxed, e.g., graphene QDs and MoS2 QDs. Below list a few reviewer papers about this topic.
• Quantum dots derived from two-dimensional materials and their applications for catalysis and energy, Chemical Society Reviews, 2016Reviews, , 45, 2239Reviews, -2262 • Two-dimensional quantum dots: Fundamentals, photoluminescence mechanism, and their energy and environmental applications, Materials Today Energy, 2018, 10, 222-240 • A critical review on two-dimensional quantum dots (2D QDs): From synthesis toward applications in energy and optoelectronics, Progress in Quantum Electronics, 2019, 68, 100226 In the original manuscript, we followed the expression in these review papers and described our C3N4QDs as 2D QDs and with 2D morphology (they are not equiaxed with lateral/thickness ratio of 5-10). After considering the Reviewer's comments, we agree that 2D QDs are strictly not an accurate terminology. Our materials are more like nanoplates, or nanotiles with quantum effects.
In the revised manuscript, we use "quantum nanotiles" to replace "2D QDs" and use "layered structure" to replace "2D morphology. We believe that these are more precise expressions. We would like to thank the Reviewer again for helping us clarify the confusing expression. Response: We sincerely thank the reviewer to mention this critical point, which actually further strengthens the importance of the dynamic reconstructed process. The percentage of the Zn ions that are influenced by C3N4QDs was calculated based on a simplified model, where the diameter of the C3N4QDs is 10 nm and their thickness is 1.8 nm (based on the TEM and AFM analysis); each structural pore on C3N4QDs can adsorb one Zn 2+ ion. As listed in Supplementary Table 2, even in this ideal situation, there will be only ~0.063% of Zn 2+ ions coordinated with C3N4QDs in electrolyte with 0.5 mg mL -1 C3N4QDs.
This extremely small percentage suggests that the impact of C3N4QDs on the overall Zn 2+ solvation structure would not be significant without the formation of dynamic protective interphase. If we look at the distribution of the Zn 2+ ions across the Zn symmetric cell at a certain time, the ones coordinated with C3N4QDs will be highly concentrated on the plating electrode surface where the protective interphase lies (Figure 3e), and the ones in the bulk electrolyte (not near the surface) will be mostly "free" (not coordinated with C3N4QDs). The C3N4QDs then constructed interphase that keeps the pores for ion-sieving open to enable water-free, single Zn 2+ ion conduction, that is, all of the deposited Zn atom (in the form of ions) must be coordinated with C3N4QDs protective interphase right before reaching the electrode and being reduced into Zn metal.
Hence the key role of the C3N4QDs additive is the construction of protective interphase upon the Zn anode.
The relevant discussion has been added to the revised manuscript. (Page 10, line 18-

23)
Supplementary However, at the beginning of each plating/stripping process (when the current just rotates directions), the C3N4QDs protective interphase on one electrode will redisperse in the electrolyte, and migrate to the other electrode and form a protective interphase. This process involves the movement of C3N4QDs that are coordinated with numbers of Zn 2+ . As the Reviewer mentioned, compared to free Zn 2+ ions solvated with water, the transport of the coordinated object would most likely be more sluggish. That is a clear disadvantage. The advantage is that a large amount of Zn 2+ undergoes synchronous migration alongside the C3N4QDs under the electrical field.
All the Zn 2+ ions on one C3N4QD reach the electrode at the same time in a welldistributed mode (following the pattern of periodic coplanar zincophilic pores in C3N4QD). Such a transport behavior ensures a generous and homogeneous Zn 2+ ion flux. The overall effect will be an interplay between these two factors. It needs to be emphasized that the formation of the protective interphase is complete in minutes judged by the luminescence microscope analysis. Most of the time in a cycle, C3N4QDs are immobilized in the interface. Hence, the sluggish transport of coordinated objects in the first few minutes has a limited impact on the overall plating of Zn. That may explain the smooth and none porous deposition of Zn.
After the C3N4QDs accumulate and construct the protective interphase on the surface of Zn anode, it serves as an ion-sieving film that prevents the reduction of solvated water and anions. The transference number (tZn2+) was calculated to quantitatively describe the Zn 2+ conducting ability of the C3N4QDs protective interphase. A rather low tZn2+ of 0.577 was obtained in the Zn symmetric cell under pure ZnSO4 electrolyte owing to the faster migration speed of the anions than solvated Zn 2+ , which is consistent with a previous report. tZn2+ can be dramatically improved to 0.796 after the formation of C3N4QDs interphase, (Supplementary Figure 9, Table 1). The structural periodic coplanar zincophilic pores in C3N4QDs provide the active sites or solvating groups for ion transfer, and the dense C3N4QDs interphase can block solvated water and anions from diffusing through this interphase. The high tZn2+ eliminates the large Zn 2+ concentration gradient and facilitates uniform ion distribution, resulting in homogeneous Zn plating.
The relevant discussion has been added to the revised manuscript. (

Response:
That is a good question. Indeed, the original texture of the Zn foil may dominate the results of many X-ray-based analyses, e.g., XRD. To avoid that, we utilized 2D synchrotron grazing-incidence X-ray diffraction (GIXD) to study the Zn foil electrodes after various plating/stripping processes. The advantage of GIXD technique on flat Zn electrodes is the limited penetration depth of the X-rays into the samples, with the benefit of low background scattering from the substrate. By varying the incident angle, the X-rays' penetration depth can be changed from a few nanometers up to 100 nanometers. As for our experiment, the GIXD patterns were measured in the incident angle range of 0.2° to 2.6°, and the ones measured at 0.8° are presented in Fig 4. The beam size is ~5 × 7 μm to collect the surface texture of Zn electrode that gets rid of the influence of bulk Zn foil. The scattered X-rays at 2θ angular range of 6-73° are recorded by a 2D X-ray sensitive detector.
As can be seen from the SEM analysis ( Figure 4i-4o), the surface of Zn electrode turns to be relatively rough and porous after the plating/stripping processes. Hence the X-rays' penetration depth enlarged and the intensity of Zn scattering signal increased sharply. The scattering signal of the C3N4QD is not as evident as that obtained after 1 st plating, however, it still can be recognized if compared with that of Zn electrode obtained in pure ZnSO4 electrolyte.

WRT the cathode, it is interesting that the cathode c-axis shrinks upon intercalation.
Could the protons be intercalated, which is smaller than Zn 2+ , but also introduces some positive charges. Is it possible to intercalate large cations Zn 2+ while having an even smaller c-axis lattice parameter? Does CNQD accumulate on the cathode (given that the authors assume a part of Zn 2+ is coordinated with CNQD, meaning Zn 2+ flux towards the cathode upon initial discharge should also deposit some CNQD?)?
Response: Many thanks. Actually, the reviewer points out one of the most critical challenges for the cathode materials in aqueous batteries, which is the protonation (ACS Sustainable Chem. Eng. 2021, 9, 8, 3223-3231 , 2020, 32, 1908140), the formation of C3N4QDs interphase might maintain a rigid structure of VOPO4 during insertion and deinsertion, hence preserving a stable cycling performance.
We would like to thank the reviewer for bringing up the potential impact on cathodes.
From the XPS results and the information in the literature, we believe that the  morphologies are typically used to describe thin (few-layer) materials with large lateral sizes, such as a piece of few-layer graphene with a lateral to axial (thickness) size ratio of over 10,000.
The situation becomes interesting when the lateral size of a piece of 2D material (e.g., graphene) was reduced to several nanometers, which is in the range of the size of quantum dots and therefore often called quantum dots. As the lateral size/thickness ratio decreases to ~10, technically the resulting materials are not typical 2D morphology anymore. However, the terminology of "2D quantum dots" is widely In the original manuscript, we followed the expression in these review papers and described our C3N4QDs as 2D QDs and with 2D morphology (they are not equiaxed with lateral/thickness ratio of 5-10  In the revised manuscript, we use "quantum nanotiles" to replace "2D QDs" and use "layered structure" to replace "2D morphology. We believe that these are more precise expressions. We would like to thank the Reviewer again for helping us clarify the confusing expression.
The relevant discussion has been added to the revised manuscript.

MD simulation was carried out to demonstrate the changes of solvation structure of Zn 2+ . How does C3N4QDs with an average lateral size of ~10 nm enter primary solvation shell of Zn 2+ ? And what is the basis of molecular models of C3N4QDs?
Related reference should be supplemented to confirm this conclusion. The changes of Zn 2+ solvation structure may be considered as large C3N4QDs entering a much smaller primary solvation shell of Zn 2+ . But a more precise description would be that the intrinsic subnanometric pore in C3N4QD (more specifically the N atoms around a pore) enter the primary solvation shell of Zn 2+ .

Response
The relevant discussion has been added to the revised manuscript. The misarrangement occurs because we first organized them by the order of 2 M ZnSO4 + 0.1 mg ml -1 C3N4QDs, 2 M ZnSO4 + 0.5 mg ml -1 C3N4QDs, 2 M ZnSO4 + 1 mg ml -1 C3N4QDs, 2 M ZnSO4 + 2 mg ml -1 C3N4QDs, 2 M ZnSO4 + 4 mg ml -1 C3N4QDs and the pristine 2 M ZnSO4, after reconsidering, we arranged the i-t curves of pristine 2 M ZnSO4 from the end to the first, but forgot to arrange the second column of EIS plots. We have provided the original misarranged Supplementary     Sci., 2022Sci., , 15, 1106Sci., -1118. The Zn|C3N4QDs|MnO2 exhibits slightly enhanced capacity retention. As for the VOPO4-based full cells, a high capacity of ~100 mAh g -1 can be achieved at the current density of 0.1 A g -1 , attributing to the major capacity contribution stemming from the diffusion-controlled process. The Zn|C3N4QDs|VOPO4 present good capacity retention with the increase of current as well as over cycling. After harsh 75 cycles, it can be noted that the Zn|C3N4QDs|VOPO4 shows a recovered capacity of 115 mAh g -1 when the current density is set back to 0.1 A g -1 . An obvious activation process and a stable profile were obtained. In contrast, the Zn||VOPO4 gradually downgraded, which is in line with the cycling results in Figure 6d. This might imply that the C3N4QDs also play a positive role in advancing the deficiencies of decomposition/dissolution of VOPO4 cathode in aqueous electrolytes. The C3N4QDs could accumulate on VOPO4 cathode to form protective cathode-electrolyte interphase, similar to the organic carbonaceous interphases upon an aqueous lithium cathode (Adv. Mater. 2020, 32, 2004017), facilitating stable interface chemistry and substantially inhibiting water decomposition on the cathode surface, hence restraining the decomposition/dissolution of VOPO4 cathode. Whereas for pristine VOPO4 cathode, the serious decomposition/dissolution of VOPO4 renders the Zn||VOPO4 full cell drastically attenuated. In the following study, we will provide an in-depth, mechanistic understanding of the roles of C3N4QDs on VOPO4 cathode, aiming to expedite the application of next-generation aqueous Zn-ion batteries.
The relevant discussion has been added to the revised manuscript. To further demonstrate the role of C3N4QDs in ZnSO4 electrolyte-based full cells, we also employed a V2O5 cathode with ZnSO4 electrolyte to fabricate a Zn||V2O5 full cells in the revised manuscript. As displayed in Supplementary Figure 32, both Zn||V2O5 and Zn|C3N4QDs|V2O5 cells exhibit a gradual increase in capacity in the initial tens of cycles, which is accordant with many previous reports and is related to the activation of the Zn 2+ intercalation process. The initial discharge capacity of Zn|C3N4QDs|V2O5 cell is 153 mAh g -1 , which increases to the maximum of 170 mAh g -1 at the 80 th cycle and remains 112 mAh g -1 after 500 cycles, 85 mAh g -1 after 1000 cycles. In contrast, the Zn||V2O5 cell can only retain a capacity of 80 mAh g -1 after 500 cycles, 52 mAh g -1 after 1000 cycles.
Therefore, we prepared 3 M aqueous Zn(OTf)2 electrolyte with 0.5 mg ml -1 C3N4QDs for a VOPO4-based full cell test. The Zn|C3N4QDs|VOPO4 full cell witnesses miraculously stable operation over 3000 cycles at 1 A g -1 with a capacity retention of 86.1% (Figure 6d). But as the reviewer mentioned, the electrolyte is a vital component that plays important role in electrochemical behavior, and different electrolyte means different property. However, we feel that the VOPO4 cathode system could demonstrate the diversity of the C3N4QDs and the universal reconstructed SEI mechanism in an aqueous electrolyte. If the reviewer feels that it is improper to include a full cell with Zn(OTf)2 electrolyte, we could delete the part of the VOPO4